Aliasing is the replacing of one recipient address with one or more
different recipient addresses. The replacement address can be that of
a single user, a list of recipients, a program, a file, or any mixture
of these. In this chapter we cover the aliases(5) file, one of
the three methods of aliasing available with the sendmail
program. We cover the other two forms,
including separate files from within the aliases file) and
~/.forward (the user's personal
:include: file) in the
Aliasing can be used to handle several complex delivery problems:
Delivering mail to a single user under a variety of usernames
Distributing a mail message to many users by specifying only a single recipient name
Appending mail to files for archival and other purposes
Filtering mail through programs and shell scripts
All the information that is needed to perform these tasks is contained in the aliases(5) file (which is often also stored in database format to make lookups faster).
The aliases(5) file is one of several sources that can supply system mail aliases. We describe it first because it is the most traditional and because it illustrates the syntax and limitations common to all techniques.
The aliases(5) file is composed of lines of text. Any line that
begins with a
# is a comment and is ignored.
Empty lines (those that contain only a newline character)
are also ignored. Any line that begins with a space or a tab is
joined (appended) to the line above it.
All other lines of text
are viewed as alias lines. The format for an alias line is:
local must begin a line. It is an address in the
form of a local recipient address (we will discuss this in more detail soon).
The colon follows the
local on the same line and may
be preceded with spaces or tabs. If the colon is missing,
sendmail prints and syslog(3)'s the following error message
and skips that alias line:
alias (to the right of the colon)
is one or more addresses on the same line.
Indented continuation lines are permitted. Each address
should be separated from the next by a comma and optional space characters.
A typical alias looks like this:
root: jim, sysadmin@server, gunther indenting whitespace
root is the local address to be aliased.
When mail is to be locally delivered to
root, it is looked
up in the aliases(5) file. If found,
is replaced with the three addresses shown earlier, and mail is
instead delivered to those other three addresses.
This process of looking up and possibly aliasing local recipients
is repeated for each recipient until no more aliases are found in the
That is, for example, if one of the aliases for
jim also exists to the left of a colon in the aliases
file, he too is replaced with his alias:
The list of addresses to the right of the colon may be mail addresses
(such as gunther or jim@otherhost),
the name of a program to run (such as /etc/relocated),
the name of a file onto which to append (such as /usr/share/archive), or the
name of a file to read for additional addresses (using
:include: is used in creating mailing lists and will be covered
in the next chapter.
The location of the aliases(5) file is specified with the
ServiceSwitchFile option (see Section 34.8.61, ServiceSwitchFile)
A) option (see Section 34.8.1, AliasFile (A))
in the configuration file. Be aware that, since these two options interact,
it may not suffice to simply declare one or the other. Also
be aware that some systems supply service-switch files that will
be used even if the
ServiceSwitchFile option is omitted.
Note that the service-switch file merely specifies the order in which various
methods should be used to look up aliases, not the specific files.
If it lists
files as a method:
then all the files declared with the
AliasFile option will be looked up in the order in which they were declared:
AliasFile option specifies a file and if
a service-switch file omits the
AliasFile option is ignored.
AliasFile option specifies a file and if
a service-switch file omits the
AliasFile option is used.
AliasFile option specifies a file and if there
is no service-switch file, then the
AliasFile option file
AliasFile option is omitted
and if there is no service-switch file or if there is a
service-switch file but it omits an
sendmail silently presumes that it should not do aliasing.
Note that service-switch files and
can list other techniques for obtaining aliases in addition to, or
instead of, an aliases(5) file. But this can lead to a side effect.
For example, if your configuration file declares
and if the service-switch file
aliases line specifies:
aliases nis files
then sendmail looks up aliases first with nis, then in the /etc/aliases file, then with nis a second time.
local part of an alias must be
in the form of a local recipient.
This restriction is enforced each time sendmail
reads the aliases(5) file. For every name
to the left
of a colon that it finds, sendmail performs the following
normalization and verification steps.
To begin, sendmail normalizes each address by removing everything but the address part. For example, consider the following two alias lines:
george (George Washington): gw George Washington <george>: gw
When sendmail reads these lines, it normalizes each into its address part:
george (George Washington) becomes george George Washington <george> becomes george
After the address part is extracted, it is converted
to lowercase and rewritten by rule sets 3 and 0 to see
whether it causes the
local delivery agent to be selected
or, beginning with V8.7 sendmail, to see whether it causes any delivery
agent with the
F=A flag set (see Section 30.8.12, F=A)
to be selected.
Here, the address
george (after processing)
selects a local delivery agent, and so
these alias lines are legal.
Internally (or in its database), sendmail stores the above
When mail arrives that is addressed for delivery to
sendmail rewrites that address with rule sets 3 and 0.
Rule set 0 selects the
local delivery agent (or, for V8.7, any
F=A set). Only if
a local delivery agent is selected for an address does
sendmail look up an address in its aliases file.
george is looked up and replaced with
Internally, sendmail marks the recipient
as defunct, having been replaced with an alias, and then adds
to the list of recipients.
The new recipient,
gw, is then processed for delivery.
Rule sets 3 and 0 are called once more and again select a local
delivery agent. As a consequence,
gw is also looked up.
If it is found to the left of a colon in the aliases file,
it too is replaced with yet another address (or addresses).
This process repeats until no new local addresses are found.
george is marked defunct rather
than being deleted to detect alias loops. To illustrate, consider
the following two mutually referencing aliases:
george: gw gw: george
The sendmail program first replaces
george as defunct. It goes to mark
as defunct but notices that a loop has been formed.
If sendmail is running in verbose mode (see Section 34.8.76, Verbose),
aliasing/forwarding loop broken
and bounces the message.
Note that aliases can get pretty complex. As a consequence, when one address aliases to many new addresses, this autodetection of loops will fail (but the problem will be caught later with "hop counting"; see Section 34.8.36, MaxHopCount (h)).